There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biochemical information. Techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, etc., are being used in the fabrication of these microfluidic systems. The term, xe2x80x9cmicrofluidicxe2x80x9d, refers to a system or device having channels and chambers which are generally fabricated at the micron or submicron scale, e.g., having at least one cross-sectional dimension in the range of from about 0.1 xcexcm to about 500 xcexcm. Early discussions of the use of planar chip technology for the fabrication of microfluidic systems are provided in Manz et al., Trends in Anal. Chem. (1990) 10(5):144-149 and Manz et al., Avd. in Chromatog. (1993) 33:1-66, which describe the fabrication of such fluidic devices and particularly microcapillary devices, in silicon and glass substrates.
Applications of microfluidic systems are myriad. For example, International Patent Appln. WO 96/04547, published Feb. 15, 1996, describes the use of microfluidic systems for capillary electrophoresis, liquid chromatography, flow injection analysis, and chemical reaction and synthesis. U.S. Pat. No. 5,942,443, filed Jun. 28, 1996, and incorporated herein by reference, discloses wide ranging applications of microfluidic systems in rapidly assaying large number of compounds for their effects on chemical, and preferably, biochemical systems. The phrase, xe2x80x9cbiochemical system,xe2x80x9d generally refers to a chemical interaction which involves molecules of the type generally found within living organisms. Such interactions include the full range of catabolic and anabolic reactions which occur in living systems including enzymatic, binding, signaling and other reactions. Biochemical systems of particular interest include, e.g., receptor-ligand interactions, enzyme-substrate interactions, cellular signaling pathways, transport reactions involving model barrier systems (e.g., cells or membrane fractions) for bioavailability screening, and a variety of other general systems.
Many methods have been described for the transport and direction of fluids, e.g., samples, analytes, buffers and reagents, within these microfluidic systems or devices. One method moves fluids within microfabricated devices by mechanical micropumps and valves within the device. See, Published U.K. Patent Application No. 2 248 891 (Oct. 18, 1990), Published European Patent Application No. 568 902 (May 2, 1992), U.S. Pat. Nos. 5,271,724 (Aug. 21, 1991) and 5,277,556 (Jul. 3, 1991). See also, U.S. Pat. No. 5,171,132 (Dec. 21, 1990) to Miyazaki et al. Another method uses acoustic energy to move fluid samples within devices by the effects of acoustic streaming. See, Published PCT Application No. 94/05414 to Northrup and White. A straightforward method applies external pressure to move fluids within the device. See, e.g., the discussion in U.S. Pat. No. 5,304,487 to Wilding et al.
Still another method uses electric fields to move fluid materials through the channels of the microfluidic system. See, e.g., Published European Patent Application No. 376 611 (Dec. 30, 1988) to Kovacs, Harrison et al., Anal. Chem. (1992) 64:1926-1932 and Manz et al. J. Chromatog. (1992) 593:253-258, U.S. Pat. No. 5,126,022 to Soane. Electrokinetic forces have the advantages of direct control, fast response and simplicity. However, there are still some disadvantages. For maximum efficiency, it is desirable that the subject materials be transported as closely together as possible. Nonetheless, the materials should be transported without cross-contamination from other transported materials. Further, the materials in one state at one location in a microfluidic system should remain in the same state after being moved to another location in the microfluidic system. These conditions permit the testing, analysis and reaction of the compound materials to be controlled, when and where as desired.
In a microfluidic system in which the materials are moved by electrokinetic forces, the charged molecules and ions in the subject material regions and in the regions separating these subject material regions are subjected to various electric fields to effect fluid flow.
Upon application of these electric fields, however; differently charged species within the subject material will exhibit different electrophoretic mobilities, i.e., positively charged species will move at a different rate than negatively charged species. In the past, the separation of different species within a sample that was subjected to an electric field was not considered a problem, but was, in fact, the desired result, e.g., in capillary electrophoresis. However, where simple fluid transport is desired, these varied mobilities can result in an undesirable alteration or xe2x80x9celectrophoretic biasxe2x80x9d in the subject material.
Without consideration and measures to avoid cross-contamination, the microfluidic system must either widely separate the subject materials, or, in the worst case, move the materials one at a time through the system. In either case, efficiency of the microfluidic system is markedly reduced. Furthermore, if the state of the transported materials cannot be maintained in transport, then many applications which require the materials to arrive at a location unchanged must be avoided.
The present invention solves or substantially mitigates these problems of electrokinetic transport. With the present invention, microfluidic systems can move materials efficiently and without undesired change in the transported materials. The present invention presents a high throughput microfluidic system having direct, fast and straightforward control over the movement of materials through the channels of the microfluidic system with a wide range of applications, such as in the fields of chemistry, biochemistry, biotechnology, molecular biology and numerous other fields.
The present invention provides for a microfluidic system which electroosmotically moves subject material along channels in fluid slugs, also termed xe2x80x9csubject material regions,xe2x80x9d from a first point to a second point in the microfluidic system. A first spacer region of high ionic concentration contacts each subject material region on at least one side and second spacer regions of low ionic concentration are arranged with the subject material regions of subject material and first or high ionic concentration spacer regions so that at least one low ionic concentration region is always between the first and second points to ensure that most of the voltage drop and resulting electric field between the two points is across the low ionic concentration region.
The present invention also provides for a electropipettor which is compatible with a microfluidic system which moves subject materials with electroosmotic forces. The electropipettor has a capillary having a channel. An electrode is attached along the outside length of the capillary and terminates in a electrode ring at the end of the capillary. By manipulating the voltages on the electrode and the electrode at a target reservoir to which the channel is fluidly connected when the end of the capillary is placed into a material source, materials are electrokinetically introduced into the channel. A train of subject material regions, high and low ionic concentration buffer or spacer regions can be created in the channel for easy introduction into the microfluidic system.
The present invention further compensates for electrophoretic bias as the subject materials are electrokinetically transported along the channels of a microfluidic system. In one embodiment a channel between two points of the microfluidic system has two portions with sidewalls of opposite surface charges. An electrode is placed between the two portions. With the voltages at the two points substantially equal and the middle electrode between the two portions set differently, electrophoretic forces are in opposite directions in the two portions, while electroosmotic forces are in the same direction. As subject material is transported from one point to the other, electrophoretic bias is compensated for, while electroosmotic forces move the fluid materials through the channel.
In another embodiment a chamber is formed at the intersection of channels of a microfluidic system. The chamber has sidewalls connecting the sidewalls of the intersecting channels. When a subject material region is diverted from one channel into another channel at the intersection, the chamber sidewalls funnel the subject material region into the second channel. The width of the second channel is such that diffusion mixes any subject material which had been electrophoretically biased in the subject material region as it traveled along the first channel.
In still a further embodiment, the present invention provides a microfluidic system and method of using that system for controllably delivering a fluid stream within a microfluidic device having at least two intersecting channels. The system includes a substrate having the at least two intersecting channels disposed therein. In this aspect, the one of the channels is deeper than the other channel. The system also includes an electroosmotic fluid direction system. The system is particularly useful where the fluid stream comprises at least two fluid regions having different ionic strengths.
The present invention also provides a sampling system using the electropipettor of the invention. The sampling system includes a sample substrate, which has a plurality of different samples immobilized thereon. Also included is a translation system for moving the electropipettor relative to said sample substrate.
The invention as hereinbefore described may be put into a plurality of different uses, which are themselves inventive, for example, as follows:
The use of a substrate having a channel, in transporting at least a first subject material from at least a first location to a second location along the channel, utilizing at least one region of low ionic concentration which is transported along the channel due to an applied voltage.
A use of the aforementioned invention, in which the ionic concentration of the one region is substantially lower than that of the subject material.
A use of the aforementioned invention, wherein a plurality of subject materials are transported, separated by high ionic concentration spacer regions.
The use of a substrate having a channel along which at least a first subject material may be transported, in electrophoretic bias compensation, the channel being divided into a first and a second portion, in which the wall or walls of the channel are oppositely charged, such that electrophoretic bias on the at least first subject material due to transportation in the first portion is substantially compensated for by electrophoretic bias due to transport in the second portion.
A use of the aforementioned invention in which a first electrode is located at a remote end of the first portion, a second electrode is located at the intersection between the portions and a third electrode is located at a remote end of the second portion.
A use of the aforementioned invention, in which the substrate is a microfluidic system.
A use of the aforementioned invention in which the substrate is an electropipettor.
A use of the aforementioned invention, in which the electropipettor has a main channel for transportation of the subject material and at least one further channel fluidly connected to the main channel from which a further material to be transported along the main channel is obtained.
A use of the aforementioned invention, in which the further material is drawn into the main channel as a buffer region between each of a plurality of separate subject materials.
The use of a microfluidic system having at least a first and a second fluid channel which intersect, in optimizing flow conditions, the channels having different depths.
A use of the aforementioned invention in which one channel is between 2 to 10 times deeper than the other channel.
The use of a microfluidic system having a first channel and a second channel intersecting the first channel, in electrophoretic compensation, the intersection between the channels being shaped such that a fluid being transported along the first channel towards the second channel is mixed at the intersection and any electrophoretic bias in the fluid is dissipated.